Heat insulation is usually selected for a particular temperature range to be encountered. At very high temperatures, the insulation must be incombustible. At intermediate temperatures, as used in a building, the insulation is chosen to be adaptable to the type of structure. At low cryogenic temperatures maximum insulation effectiveness is desired, as it is more costly to remove heat than to replace it, as is the case at elevated temperatures. In the insulating of cryogenic structures the combination of the insulation and structure should be optimized. Although the insulation set forth hereinafter is primarily directed towards the cryogenic range, it may become economical for installations on structures, experiencing higher temperatures as fuel prices increase.
Large cryogenic tanks, such as for liquid natural gas tanks, often referred to as LNG tanks, have used a variety of insulations, including fiberglass, plastic foams, and balsa wood.
At the temperature of LNG, -165.5.degree. C., the insulation requirements are less demanding than at the extremely low temperatures of liquid oxygen, nitrogen, helium, and hydrogen. For such low temperatures evacuated powders and multiple radiation shields spaced with low density fibers have been researched for use in small tanks for space vehicles. Conductivity of 1/100th of that of conventional foam insulations has been achieved. However application of this technology has not been applied to the large tanks used for transportation of liquid natural gas. Many large tank designs impose compressive loads on the insulation. Whereas a wall may have highly effective insulation between structural supports, the latter often constitute large heat leaks.
Considering the tank wall with its structure and insulation as a whole, the accepted figure of merit is the allowable compressive stress divided by the heat conductivity. If the insulation is capable of taking the loads by itself, the total heat transfer can be less than with a composite wall. Balsa will take the weight of a large LNG tank as used on many ships transporting liquid natural gas. However a balsa insulated LNG tank is usually stronger than required, and does not have particularly low conductivity. Plastic foams are often used on the sides of tanks where loads are lighter. However in respect to plastic foams, there is a minimum possible heat conductance. As the density of plastic foam insulation is decreased, the ability decreases to block radiative and convective heat, and the compressive strength may be lowered below practical utility. In contrast, in using the hereinafter described heat insulation, it is possible in designing LNG tanks to design separately for structural loads, and radiative and convective heat transfers.